This invention relates to a composition useful as insulating conductors. The composition consists essentially of an ethylene-vinyl acetate copolymer, a zinc salt, aluminum trihydrate, and a peroxide curing agent. This insulating composition surprisingly provides excellent long-term thermal stability with fast cure.
Many different compositions are used as polymeric insulators for electrical conductors. These compositions typically contain a polymer or copolymer, such as polyethylene or ethylene-vinyl acetate copolymer. However, these polymers alone are ineffective insulators due to oxidative degradation of the polymeric material at the higher temperatures usually found in electrical devices.
Because of the instability of the polymers as insulators, various additives are typically mixed with the polymeric materials to impart stabilization. Standard additives useful for coating materials for electrical conductors include hindered phenol antioxidants such as Irganox 1010 or Irganox 1035. Other additives that are added to the insulating materials include zinc salts, peroxide curing agents, antimony oxide, lead compounds, and others. Usually, the insulators contain a large number of these additives in their formulation. See for example, U.S. Pat. Nos. 4,857,673, 4,260,661, and 3,819,410. However these additives, in particular the phenol antioxidants, are expensive and add additional cost to the insulation.
Another typical problem encountered in developing suitable insulating compositions is that the cure rates for these compositions are often too slow for certain applications. For instance, fast cure rates are desirable in applications where high processing line speed is necessary, particularly in the automotive wire application area. It would thus be useful to find a composition that would allow for fast cure rates.
In sum, new insulating compositions for electrical conductors are needed. Particularly valuable insulating compositions would have improved heat aging properties and fast cure for higher processing line speeds. Ideally, the new compositions would also have a minimal amount of additives in order to lower formulation cost.
The invention is an insulating composition consisting essentially of a copolymer of ethylene and vinyl acetate, a zinc salt of a mercaptobenzimidazole, aluminum trihydrate, and a peroxide curing agent consisting of para and meta isomers of xcex1,xcex1xe2x80x2-bis(t-butylperoxy) diisopropyl benzene. It is surprisingly found that the insulating composition has improved thermal stability and fast cure.
The insulating composition of the invention consists essentially of a copolymer of ethylene and vinyl acetate and a number of additives. The additives are a zinc salt of a mercaptobenzimidazole, aluminum trihydrate, and a peroxide curing agent consisting of para and meta isomers of xcex1,xcex1xe2x80x2-bis(t-butylperoxy) diisopropyl benzene. The composition has improved heat-aging and fast cure properties.
Copolymers of ethylene and vinyl acetate are well known in the art. Ethylene-vinyl acetate copolymers preferably contain between about 5 and 35 weight percent vinyl acetate, most preferably between about 15 and 25 weight percent vinyl acetate.
Ethylene-vinyl acetate is mixed with various additives to form the insulating composition of the invention. The amount of ethylene-vinyl acetate copolymer useful in the composition ranges from about 30 to about 65 weight percent of the entire weight of the composition mixture, and preferably from about 40 to about 50.
The inventive composition also contains a zinc salt of a mercaptobenzimidazole. Zinc salts of a mercaptobenzimidazole are well known in the art. Preferably, the zinc salt of a mercaptobenzimidazole has the formula 
where R is a C1-4 alkyl group and n is 0 to 4. Most preferably, the zinc salt of a mercaptobenzimidazole is zinc mercaptotolyl imidazole. The amount of zinc salt useful in the composition ranges from about 0.5 to about 6.0 weight percent of the entire composition mixture, and preferably from about 1.5 to about 3.0 weight percent.
Aluminum trihydrate-is also contained in the insulating composition. The aluminum trihydrate is useful as a flame-retardant filler for the composition. The aluminum trihydrate accounts for approximately 30 to about 70 weight percent of the composition mixture, and preferably from about 45 to about 55 weight percent.
The insulating composition of the invention also contains a peroxide curing agent consisting of para and meta isomers of xcex1,xcex1xe2x80x2-bis(t-butylperoxy) diisopropyl benzene. The peroxide curing agent is useful for cross-linking the polymer. The peroxide curing agent component can vary in amount from about 0.2 to about 2.0 weight percent of the composition mixture, and preferably from about 0.4 to about 1.0 weight percent.
The composition of the invention may also contain pigments, lubricants, and processing aids provided that they do not interfere with cross-linking or detract from the physical properties of the composition. A processing aid can also be added to the mixture to facilitate extrusion. Preferred processing aids include alkoxysilane additives. Any conventional alkoxysilane known in the art can be used as long as it does not combust or degrade during polymer processing or interfere with crosslinking. Alkoxysilanes having 2 or 3 C1-3 alkoxy substituents, e.g., methoxy, ethoxy, propoxy, or combinations thereof, are particularly advantageous. Illustrative silanes include methyltriethoxysilane, methyltris(2-methoxyethoxy)silane, dimethyldiethoxylsilane, ethyltrimethoxysilane, vinyltris(2-methoxyethoxy)silane, phenyltris(2-methoxyethoxy)silane, vinyltrimethoxysilane, vinyltriethoxysilane, and gamma-methacryloxypropyl trimethoxysilane. The alkoxysilane component, if present, can vary in amount from about 0.2 to about 3.0 weight percent of the composition mixture, and preferably from about 0.5 to about 2.0 weight percent.
The copolymer and additives are mixed using any conventional procedure. Mixing technology is well known in the prior art. For instance, an internal mixer such as a Banbury mixer can be used. Other high shear internal mixers, including Farrel continuous mixer, Boiling Mixtrumat(trademark), or Werner and Pfleiderer mixers, can also be used in the mixing procedure.
Typically, the copolymer, zinc salt, and aluminum trihydrate are first mixed together before the peroxide curing agent is added. The peroxide is then added to the first mixture under controlled temperature conditions. The temperature of peroxide mixing should be controlled in order to prevent premature cross-linking. Preferred peroxide mixing temperatures range from about 80 to about 120xc2x0 C., with 90 to 110xc2x0 C. being most preferred.
The resulting pellets are then applied to electrical conductors to form an insulating layer surrounding the conductor. The layer provides insulation and physical protection for the conductor and flame retardancy for the jacketed conductor. The composition mixture is applied using any conventional coating techniques. Coating methods are well known in the art. A typical procedure is to apply the composition by extruding a substantially uniform 2 to 100 mil thick layer onto a metal conductor. More typically, insulation thicknesses will range from 10 to 60 mils. The extrusion is carried out using a single screw extruder at the desired line speeds. Curing is typically accomplished by passing the insulated wire through a steam tube maintained at 260 psi immediately following extrusion.